The present invention relates to a thermal type air flow sensor for measuring an air flow rate employing a heating resistor. More particularly, the invention relates to a thermal type air flow sensor suitable for measuring an intake air flow rate of an internal combustion engine or so forth.
Conventionally, a thermal type air flow sensor has been used as a sensor for measuring an intake air flow rate flowing through an air intake passage of an internal combustion engine of an automotive vehicle or so forth. Such thermal type air flow sensor has been evaluated for capability of directly detecting a mass flow rate.
In the recent year, a thermal type air flow sensor fabricated by a semiconductor fine patterning technology on a semiconductor substrate, such as silicon (Si) or the like, has been attracting attention for capability of fabrication in relatively easy and by a mass production system, and of driving at low power.
As a basic principle of the thermal type air flow sensor employing such conventional semiconductor technology, there is one illustrate in FIGS. 12A and 12B, for example.
FIG. 12A is a circuit diagram of the thermal type air flow sensor and FIG. 12B is a plan view showing a layout of a heating resistor Rh and an air temperature measuring resistor Rc for measuring air flow rate.
The heating resistor Rh of shown example operates as both of an air flow rate measuring element and a heater. On the other hand, an air temperature measuring resistor Rc is used for control to maintain a temperature difference of the heating resistor and an air temperature constant even when a temperature of an intake air is varied. These resistors Rh and Rc are formed with temperature sensitive resistors having common directionality of variation of resistance values relative to a temperature. Resistance values of the heating resistor Rh and the air temperature measuring resistor Rc are set so that a large current flows through the heating resistor Rh for causing heat generation, and, in contrast, a little current not causing little heat generation flows through the air temperature measuring resistor Rc. These heating resistor Rh and the air temperature measuring resistor Rc form a bridge circuit together with fixed resistors R1 and R2. A voltage between the resistors Rh and R1 and a voltage between resistors Rc and R2 are input to an operational amplifier Op for controlling a heating current flowing through the heating resistor Rh via the operational amplifier Op and a transistor Tr so that a temperature difference between the heating resistor Rh and an air temperature (air temperature measuring resistor Rc) becomes a predetermined temperature xcex94Th. The heating current becomes a value corresponding to an air flow rate. Then, by converting this current into a voltage by the resistor R1, the air flow rate is detected.
As shown in FIG. 12B, upon fabricating the heating resistor Rh and the air temperature measuring resistor Rc by semiconductor fine patterning on a semiconductor substrate 300, the heating resistor Rh and the air temperature measuring resistor Rc are formed via an electrically insulative film (electrically insulative layer) on the semiconductor substrate 300, such as a silicon (Si) substrate or the like. However, concerning the heating resistor Rh, a part of the semiconductor substrate 300 is removed to certainly define a space (cavity portion) 301 to arrange the overall heating resistor Rh via the electrically insulative layer on the space 301 formed by removal of part of the semiconductor substrate. Thus, escape of heat of the heating resistor Rh by heat transmission through the semiconductor substrate 300 can be avoided (prevention of heat radiation other than air flow rate). On the other hand, the air temperature measuring resistor Rc is required to restrict heat generation as small as possible so as to enhance accuracy of measurement of air temperature. Therefore, the air temperature measuring resistor Rc is arranged on the semiconductor substrate 300 outside of the space 301.
FIGS. 13A and 13B are illustration showing a principle of another example of the conventional thermal type air flow sensor.
In the shown example, a temperature measuring resistor Rs heated by the heating resistor Rh (which temperature measuring resistor Rs is as it were, a temperature sensing resistor detecting a heat of the heating resistor Rh), a air temperature measuring resistor Rc and fixed resistors R1 and R2 form a bridge circuit. A voltage between the resistors Rs and R1 and a voltage between the resistors Rc and R2 are input to an operational amplifier Op1 to control a heating current flowing through the heating resistor Rh via the bridge circuit, the operational amplifier Op1 and the transistor Tr so that a temperature difference between the temperature measuring resistor Rs, thus the heating resistor Rh and the air temperature (air temperature measuring resistor Rc) is maintained at a predetermined temperature. The heating resistor Rh this managed the temperature heats a temperature measuring resistor Ru arranged upstream side of the heating resistor Rh and also a temperature measuring resistor Rd arranged downstream side of the heating resistor Rh. The temperature measuring resistors Ru and Rd form a bridge circuit together with fixed resistors R1xe2x80x2 and R2xe2x80x2. When air flow is generated, a difference of calorific values to be removed from the upstream side and downstream side temperature measuring resistors Ru and Rd depending upon air flow rate due to positional relationship thereof. By detecting the difference by an operational amplifier Op2, air flow rate can be detected.
Even in such type, the air temperature measuring resistor Rc to be used for maintaining the temperature difference between the heating resistor Rh and the air temperature at a predetermined value, is arranged outside of the cavity portion 301 formed by removing a part of substrate 300. On the other hand, all of the heating resistor Rh and the temperature measuring resistors Rs, Ru and Rd intended to be heated by the heating resistor are arranged on the cavity portion 301 via the electrically insulative layer (electrically insulative film).
As the thermal type air flow sensor utilizing the principle set forth above, there are sensors disclosed in JP-A-2-259527, JP-A-4-320927, JP-A-6-273208, JP-A-6-50783, JP-A-8-14976, JP-A-10-160538, and Tokuhyo Hei No. 10-500490.
In the prior art set forth above, sufficient consideration has not been given for an error in detection of air flow rate in the case where fouling substance, such as dust or so forth contained in the intake air, adheres or deposits on a surface of the thermal type air flow sensor. If such thermal type air flow sensor is continuously used for a relatively long period, it is expected that the initial accuracy cannot be maintained for the reason set forth above.
As fouling substances for the thermal type air flow sensor possibly contained in the intake air of the internal combustion engine may be Si, Fe, Ca, Mg, Na contained in solid particle, typically sand, NaCl, MgCl2, CaCl2 contained in snow melting agent, engine lubricant oil contained in blow-by gas, H2O, C, an impregnating oil of an air filter in a wet type air cleaner, and so forth, for example. The substances set forth above may adhere on the surface of the thermal type air flow sensor due to intermolecular attraction, liquid bridging force, electrostatic force, and composite force thereof.
Once the fouling substance adhere on the surface of the thermal type air flow sensor, thermal transmission from the heating resistor to ambient air or aspect of thermal transmission can be varied due to the adhered or deposited substance to degrade accuracy of measurement to be insufficient. Such problem can be caused even for different types of thermal type air flow sensors as illustrated in FIGS. 12A, 12B and 13.
The present invention has been worked out in view of the problems set forth above. Therefore, it is an object of the present invention to provide a thermal type air flow sensor which can correct variation of characteristics of a thermal type air flow sensor due to adhesion or deposition of fouling substance contained in an intake air and thus can maintain initial accuracy.
According to the basic construction of the present invention, a thermal type air flow sensor for measuring an air flow rate using a heating resistor and a temperature measuring resistor for measuring an air temperature, comprises:
a semiconductor substrate, a part of which is removed for defining a space therein;
the heating resistor and a portion of the temperature measuring resistor being formed above the space via an electrically insulative layer, remaining portion of the temperature measuring resistor being formed on the semiconductor substrate at a location offsetting from the space; and
means for correcting an air flow rate measurement error on the basis of a voltage of the portion of the temperature measuring resistor located above the space.
A resistance value of the temperature measuring resistor is set to be sufficiently greater than that of the heating resistor to flow extremely low current for suppressing heating. While a little heating is caused in the temperature measuring resistor by the extremely low current, since most portion of the temperature measuring resistor is offset from the space defined by removing the semiconductor substrate, heat is transmitted to the semiconductor substrate via the electrically insulative layer. Thus, the temperature measuring resistor generate little heat.
In the present invention, since the portion of the temperature measuring resistor is located above the space, the portion of the temperature measuring resistor is thermally isolated by the space. As a result, heat transmission to the semiconductor substrate from this portion is little to cause a little self-heat generation. The self-heat generation is caused in completely the same manner as heating in the heating resistor except for the current value to flow. On the other hand, since the temperature measuring resistor is in contact with the intake air in completely the same manner as the heating resistor, fouling may deposit in the similar manner as the heating resistor. Accordingly, by deposition of the fouling, variation of heat conduction and heat transmission is caused in the similar manner as the heating resistor.
When self-heating ability is provided for the portion of the temperature measuring resistor as set forth above, if the heat conductivity and heat transmitivity are varied by deposition of fouling on the temperature measuring resistor as set forth above, variation of voltage is caused associating with variation of characteristics due to variation of heating condition, and thus variation of resistance characteristics even when a given voltage is applied to the temperature measuring resistor. Accordingly, when the voltage (potential difference) of the portion (portion where heating ability is provided) of the temperature heating resistor, is detected, it becomes possible to indirectly detect characteristics variation of the heating resistor due to deposition of fouling, and also to correct the detected value of the air flow rate utilizing the voltage thus detected.
It should be noted that the temperature measuring resistor used for control for maintaining a difference of the temperature of the heating resistor and the air temperature constant, it is originally preferred to restrict heating ability. However, when heating ability is provided only for the portion of the temperature measuring resistor as in the present invention, little influence may be caused on the accuracy of air flow rate measurement. Rather, by correction of measurement error associating with variation of characteristics of the heating resistor due to deposition of fouling, it may contribute for improvement of accuracy of air flow rate.
It should be noted that as an example of heating of the temperature measuring resistor, there is a technology disclosed in JP-A-8-14976. This prior art is intended to improve response characteristics of the thermal type air flow sensor and heats the overall temperature measuring resistor. In the present invention, is differentiated from the technology disclosed in JP-A-8-14976 in that overall temperature heating resistor is heated, and a part of voltage of the temperature measuring resistor (voltage of the resistor portion of the temperature measuring resistor partially arranged above the space of the semiconductor substrate) is utilized for correction of measuring error.